Wind turbine generator systems are usually constructed in open areas or coastal areas. A height of the wind turbine generator system generally exceeds 130 meters, particularly, the height of a new-type wind turbine generator system usually exceeds 160 meters, and the wind turbine generator system is a protruding object in these areas, thus the wind turbine generator system is easily struck by lightning.
Therefore, lightning protection for the wind turbine generator system is crucially important, especially for a direct-driven wind turbine generator system in which a generator is arranged outside of a nacelle. Since cost of a pitch bearing, a generator main bearing and a yaw bearing in a generator of the direct-driven wind turbine generator system occupies a large proportion in the total cost of the direct-driven wind turbine generator system, and a lightning current may cause the three types of bearings to burn and damage, thus the lightning protection for the direct-driven wind turbine generator system is particularly important.
Currently, as shown in FIG. 1, a lightning protection method for the direct-driven wind turbine generator system includes arranging a separate cable lead in a blade, and taking a metal casting, a structural member and the bearings of the direct-driven wind turbine generator system as a conduction path for the direct lightning, that is, a down lead of a blade lightning arrester is connected to a flange at a root of the blade of the direct-driven wind turbine generator system, the flange at the root of the blade is connected to a wheel hub via the pitch bearing, the wheel hub is connected to the generator via the generator main bearing, the generator is connected to a base for the nacelle, the base for the nacelle is connected to a tower via the yaw bearing, and the tower is connected to an own structure of the direct-driven wind turbine generator system of a wind turbine base to discharge the direct lightning current. FIG. 1 is a schematic view showing a circulation path of a direct lightning current through a direct-driven wind turbine generator system in the conventional technology.
However, since there is a clearance between an inner race and an outer race of the bearing of the generator, impedance in the circulation path of the lightning current is high, which affects discharging of the lightning current. In addition, the bearings may be burned and then damaged by the lightning current in case of using the bearings as the circulation path of the lightning current, thereby increasing the total cost. Moreover, a normal operation of the direct-driven wind turbine generator system may be interrupted in the case that the generator is shut down to replace a new bearing, which further brings a huge economic loss.
A lightning protection method and a lightning protection device for the wind turbine generator system according to the conventional technology has constructed a lightning current discharging path, in which the lightning current flows from the blade, through a wheel hub cover and a nacelle cover, and then flows to the ground, thus allowing the lightning current to completely avoid flowing through the pitch bearing, and ensuring safety of the pitch bearing. However, other bearings such as the generator main bearing and the yaw bearing may still be damaged by the lightning current flowing through the bearings. In addition, although the lightning current discharging path only passes through the wheel hub cover and the nacelle cover, the lightning current may be shunted to the bearings, thereby damaging the bearings. Also, since the generator of the direct-driven wind turbine generator system is arranged outside of the direct-driven wind turbine generator system, the lightning protection issue for the generator can not be completely addressed with the above design.